The present invention relates to indirect printing systems and more particularly to compositions suitable for the treatment of intermediate transfer members.
Digital printing techniques have been developed that allow a printer to receive instructions directly from a computer without the need to prepare printing plates. Amongst such printing devices are printers with color laser technology or the xerographic process, which use dry toners, and the widely used inkjet printers, which use liquid inks and rely on inkjet or bubble jet processes. Such printing devices typically directly apply the desired image to the final printing substrate (e.g., paper, cardboard or plastic). In general, the resolution of such processes is limited. For instance, liquid inks may wick into fibrous substrates requiring the use of substrates specially coated to absorb the liquid ink in a controlled fashion or to prevent its penetration below the surface of the substrate. Such coated substrates may not address all issues associated with direct printing and may even create their own problems. For instance, if the surface of the substrate remains wet following the application of the ink, additional costly and time consuming steps may be needed to dry the ink, so that it is not later smeared as the substrate is being handled, for example, stacked or wound into a roll. Furthermore, excessive wetting of the substrate causes cockling and makes printing on both sides of the substrate (also termed perfecting or duplex printing) difficult, if not impossible.
In commercial settings, there exist additional printing systems, some relying on indirect or offset printing techniques. In such processes, an intermediate image of the final desired pattern (e.g., a mirror image) is typically formed on an image transfer member (e.g., a blanket or a drum) and transferred therefrom to the final printing substrate. The intermediate image can be, as in HP-Indigo printers, an electrostatic image produced on an electrically charged image bearing cylinder by exposure of compatible oil-based inks to laser light, the ink image being then transferred by way of a blanket cylinder onto paper or any other substrate. Though such systems are better suited for high quality digital printing the use of oil-based inks has raised environmental concerns.
The present Applicant has recently disclosed a printing process wherein inks having an aqueous carrier are jetted onto an intermediate transfer member (ITM) at an image forming station and dried thereupon before being transferred to the desired substrate at an impression station. Few systems implementing such process were disclosed, differing among other things in the number of image forming stations, the configurations of the intermediate transfer members, the number of impression stations and the system architecture allowing duplex printing. More details on such systems are disclosed in PCT Publication Nos. WO 2013/132418, WO 2013/132419 and WO 2013/132420.
Advantageously, such indirect printing systems allow the distance between the outer surface of the intermediate image transfer member (also called the release layer) and the inkjet print head to be maintained constant and reduces wetting of the substrate, as the ink can be dried on the intermediate image transfer member before being applied to the printing substrate. Consequently, the final image quality is less affected by the physical properties of the substrate and benefits from various other advantages as disclosed in PCT Publication Nos. WO 2013/132345, WO 2013/132343 and WO 2013/132340 by the present Applicant.
Among the problems surmounted by such systems was the need to find a balance between opposite requirements. On the one hand, the printing process, including the materials or formulations employed therewith, should allow transiently fixing the aqueous based ink droplets onto the release layer at the image forming station. On the other hand, the same should allow the dried ink film to be fully transferred to the printing substrate at the impression station.
Generally, silicone coated transfer members are preferred, since they facilitate transfer of the dried image to the final substrate. However, silicone is hydrophobic, which causes water based ink droplets to bead on the transfer member. This results in a small contact area between the droplets and the blanket that may renders the ink image unstable during rapid movement and may makes it more difficult to remove the water from the ink, for instance by heating the transfer member.
One solution proposed in the above-referenced publications of the Applicant to alleviate this problem was to “freeze” the shape of the impinging jetted droplet in the pancake-like form it adopted upon contact, for instance by rapidly evaporating a substantial proportion of the liquid ink carrier at the stage of the image formation onto the transfer member. The rate of such evaporation depending upon temperature, it was generally preferred for that particular purpose to operate the system at elevated temperatures (e.g., above water boiling point and typically up to 160° C.). However, as the vapors of the ink carrier might over time affect the print head nozzles, lower temperatures (e.g., above 40° C.) were also considered for the image forming station.
Alternatively, or additionally, the Applicant disclosed conditioning methods and formulations facilitating the desired interaction between ink formulations and materials composing the release layer suitable for the novel process, by pre-treatment of the transfer member ahead of ink jetting. More details on such methods can be found in PCT Publication No. WO 2013/132339.
Without detracting from the importance of these advances, the present inventors have discovered that under some conditions, surprisingly, some of the aforementioned conditioning solutions may deleteriously accumulate on the transfer member on selected areas. Hence, the present inventors have recognized the need for further improvements in release layer conditioning compositions and technologies.